Flu outbreak: Protecting schools

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Scientists use estimates to determine which strains go in a seasonal vaccine

Scientists and health officials are able to track flu outbreaks down to the county and determine much about an outbreak's severity and how the virus is spreading.

But there's still much that's unknown about influenza. Here are three things we still don't know about the flu:

When flu seasons begin and end. Seasonal flu activity can begin as early as October and continue to occur as late as May, according to the Centers for Disease Control and Prevention. The average flu season lasts for 12 consecutive weeks, or three months. The 2011-2012 flu season began late and was relatively mild, with cases remaining low through February.

In early December 2012, Dr. Thomas Frieden, CDC director, told reporters an increase in cases suggested flu season had begun. "This is the earliest regular flu season we've had in nearly a decade, since the 2003-2004 flu season," he said. However, every flu season is different.

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When flu cases peak. Generally, flu activity peaks in January or February, but there are variations. For instance, flu activity peaked twice during the 2009 H1N1 pandemic, once in the spring and again in the fall. The CDC considers the second peak, in October, to be the peak of the 2009-2010 flu season because the April peak occurred after the flu season had ended.

Frieden told reporters on Friday that officials won't know for a week or two if we are over the peak. While it does appear the peak may have hit some areas, such as the South, the flu will likely continue for the next few weeks and the virus is unpredictable.

What next year's flu strains will be. Seasonal vaccines protect against three influenza viruses that research indicates will be the most common in the upcoming season, according to the CDC. These include one influenza A virus and one influenza B virus. Based on international surveillance and scientists' estimations about which strains will circulate, the viruses in the vaccine can change each year.

Frieden said in December, "This year's strains look to be a great match with this year's vaccine."

The World Health organization recommends specific viruses be included in vaccines, but each country makes its own decision about which to include in vaccines in that nation. In the United States, the Food and Drug Administration determines which viruses will be used.

Within the next few years, vaccines will be available that have room for four strains, officials said Friday, but a four-strain vaccine is not available for the 2012-2013 season.